Diplexer Design
Understanding Diplexer Design
Diplexers enable multi-band operation through a single antenna, avoiding the cost and complexity of separate antennas for each band. The complementary filter design ensures low loss in-band and high isolation between bands.
Diplexer Topologies
- LPF/HPF junction: Complementary low-pass and high-pass filters at a common junction. Simple, wideband.
- BPF/BPF junction: Two bandpass filters at a common junction. Better for widely separated bands.
- Manifold: Multiple channels combined at a common junction. Extends diplexer to triplexer, quadruplexer, etc.
Design Challenges
- Junction matching: The junction must present matched impedance at all frequencies. Requires careful filter design.
- Guard band: Minimum frequency separation between bands. Tighter guard bands require higher-order filters.
- Power handling: Diplexers in transmitter paths must handle full TX power.
Frequently Asked Questions
What is a diplexer?
A diplexer separates or combines two frequency bands at a common port using complementary filters. It shares one antenna between two bands without the loss of a resistive splitter. Used in multi-band radios, satellite, and cellular base stations.
What is the difference between a diplexer and a duplexer?
A diplexer separates two different frequency bands. A duplexer separates the transmit and receive signals, which may overlap in frequency (using circulators instead of filters). The terms are sometimes used interchangeably but have different technical meanings.
How close can the two bands be?
Depends on filter order and technology. With 4th-order ceramic filters: 5-10% guard band. With cavity filters: 2-5% guard band. With acoustic (SAW/BAW) filters: < 1% guard band. Closer spacing requires sharper filters and more complex design.